Hard drive carriers are multipurpose. The hard drive carrier/frame/removable hard drive enclosure is just a modified drive bay that allows a standard hard drive in a carrier assembly to be removed and inserted with little effort. The carriers have a release handle that can be used to push the drive out of the server. Basically, a hard drive carrier converts an internal hard drive into a removable hard drive.
Hard drive connectors have to hit backplane connectors precisely, or the entire installation won’t work. That’s another purpose of the hard drive carrier – it physically holds the hard drive in the correct position, so the connectors meet.
It’s important to have the correct carrier for the hard drive. One hard drive should be installed per drive carrier. If the server bays aren’t full, a plastic drive “blank” cover can be used, which also helps the cooling process work properly so hard drives won’t overheat.